This invention relates to coding techniques for the digital transmission of video signals, and more particularly to high compression coding methods and apparatus for the digital transmission of higher quality video signals.
The installation of optical fibers and the trend towards a Broadband Integrated Switched Digital Network (B-ISDN) is expected to encourage the proliferation of higher quality video services such as high definition video conferencing and advanced television (ATV), the latter frequently referred to as high-definition television (HDTV). As a result, ATV transmission and other video services may become a major traffic component on B-ISDN. For efficient utilization of the available bandwidth, ATV transmission at a relatively slow rate of, for example, 45 Mb/s, demands high compression interframe video coding.
Various image compression techniques and their applicability to video signal processing have been studied and reported on in the literature in recent years. For example, there have been extensive reports in the literature relating to sub-band coding for image applications using quadrature mirror filtering QMF. Some of these articles that discuss sub-band coding of images include: "Multi-Dimensional Sub-band Coding; Some Theory and Algorithms," by M. Vetterli, Signal Processing 6, (1984), pp. 97-112; "Sub-band Coding of Images," by J. W. Woods and S. D. O'Neil, Proc. ICASP 86, pp. 1005-1008, April, 1986; "Sub-band Coding of Digital Images Using Two-Dimensional Quadrature Mirror Filtering," by the inventor herein and A. Tabatabai, Proc. SPIE, Vol. 707, pp. 51-61, September, 1986; "Sub-band Coding of Images," by J. W. Woods and S. D. O'Neil, IEEE Trans. ASSP, Vol. 34, pp. 1278-1288, October, 1986; "Applications of Quadrature Mirror Filtering to the Coding of Monochrome and Color Images," by the present inventor and A. Tabatabai, Proc. ICASP 87, Vol. 4, pp. 2384-2387; "Sub-band Coding of Digital Images using Predictive Vector Quantization," by P. H. Westernik, J. Biemond, and D. E. Boekee, Proc. ICASP 87, Vol. 3, pp. 1378-1381; "Sub-band Coding of Digital Images Using Symmetric Short Kernel Filters and Arithmetic Coding Techniques," by D. LeGall and A. Tabatabai, Proc. ICASP 88, Vol. 2, pp. 761-764; "Sub-band Coding of Color Images Using Differential Vector Quantization," by the present inventor, International Picture Coding Symposium, PCS 87, 8.H; and "Sub-band Coding of Monochrome and Color Images," by the present inventor and A. Tabatabai, IEEE Trans. on Circuits and Systems; Vol. 35, pp. 207-214, February, 1988.
As described in the literature, decomposition of the frequency spectrum of an image enables the decomposed bands to be quantized and coded independently of each other. Thus the higher frequency bands, where a large quantization noise would be subjectively less noticeable than in the lower frequency bands, can be quantized with a larger dead zone and with larger step sizes and thus fewer bits than can the lower frequency band with a concomitant overall improvement in coding efficiency.
Most of the aforenoted literature reports on sub-band coding can be classified into two categories: i) the analysis and/or synthesis of QMF filter design and ii) coding. The latter have concentrated on still images applications with very few discussing video applications. Where video applications have been considered, the video signal has been treated, for the sake of simplicity, as a sequence of independent single images as, for example, in "Transmission of HDTV Signals Under 140 Mbits/s Using a Sub-band Decomposition and Discrete Cosine Transform Coding," by D. LeGall, H. Gaggioni, and C. T. Chen, Proc. National Communications Forum, Vol. 42, pp. 1682-1684, September, 1988. Whereas these prior art techniques using sub-band decomposition have taken advantage of the intraframe redundancies in coding the video image, they have not eliminated the interframe redundancies.
Since there is a significant amount of redundancies between consecutive frames of video signals, various other coding techniques have been studied for removing these redundancies such as picture replenishment or interframe predictive coding. (See, for example, "Advances in Picture Coding," by H. M. Musmann, P. Pirsh, and H. J. Gravoert, Proc. IEEE, Vol. 73, pp. 523-548, April 1985). Many applications of high compression video coding involve the use of hybrid coding which is described in "An Adaptive Strategy for Hybrid Image Coding," by A. Habibi, IEEE Trans. Commun., Vol. COM-29, pp. 1736-1740, December, 1981; and "Progress of CCITT Standardization on n.times.384 Kbits/s Video Codec," by S. Okubo, R. Nicol, B. Haskel, S. Sobri, Globecom-87.
The hybrid coding method, which is a combination of DPCM and transform coding, is presently considered the most effective coding for video teleconferencing applications. A disadvantage of this method, however, is the subjective degradation in the decoded image whereby the viewers can perceive the outlines of the blocks. This latter problem can be alleviated somewhat by employing a block subdivision technique described by the present inventor in "Low Bit-Rate Video Transmission for ISDN Application," IEEE Trans. Circuits and Systems, Vol. 35, pp. 258-261, February, 1988, and which also is the subject of U.S. Pat. No. 4,821,119 issued on Apr. 11, 1989 to the present inventor. Although this latter method improves the block distortion by reducing the block size, block distortion is not totally eliminated.
An object of the present invention is to achieve significantly high video compression by removing both interframe and intraframe redundancies in the digital processing of video signals.
An additional object of the present invention is to eliminate the block distortion that is inherent in prior art hybrid coding techniques.
A further object of the present invention is to entropy encode the video information in as efficient manner as possible and thereby transmit the digital video signal with a desirable high compression ratio.